Application to Industrial Crystallizers

Secondary nucleation is a complex phenomena and due to the various competing mechanisms present it is difficult to predict the nucleation rates. However, general recommendations can be made to control nucleation rates and hence, the final crystal size distribution. The possible sources of secondary nuclei in an industrial crystallizer are given in Table 2.8 along with suggestions for preventing excessive nucleation (Jancic and Grootscholten 1983). 

In the industrial crystallizer, primary homogeneous nucleation is not possible due to the low levels of supersaturation employed and the presence of impurities. Primary heterogeneous nucleation is possible at places where high supersaturation is being created— for example, in heat transfer equipment. This could lead to crystal deposits on these surfaces. Undesirable nucleation can be reduced by choosing proper production rates and by allowing adequate mixing. 

Source of Nuclei Type of Nucleation Process Prevention or Remedy 

Boiling zone Primary Reduce specific production rates, increase crystal surface area 

Hot feed inlet Primary Enhance heat dissipation, reduce degree of superheating, carefully chosen inlet position 

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If the concentration of a solution can be measured at a given temperature, and the corresponding equilibrium saturation concentration is known, then it is a simple matter to calculate the supersaturation (equations 3.67-3.69). Just as there are many methods of measuring concentration (section 3.9.2) so there are also many ways of measuring supersaturation, but not all of these are readily applicable to industrial crystallization practice. 

These fundamental equations apply to many systems involving discrete entities aerosols, molecules, and particles, even people. A full review of their derivation of these equations is to be found in Randolph and Larson (1988), who have pioneered their application to industrial crystallizers in particular. 

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